Heretofore the most commonly utilized internal combustion engines have generally utilized vaporized liquid petroleum fuel for motive power, have often controlled inlet air and fuel ratios to achieve stoichiometric balance for engine air-fuel combustion, and typically now require adjunct emission control equipment to reduce engine NO.sub.x emissions to below required minimum levels. Often such engines have utilized carburetor by-pass air features to control to a desired constant air/fuel ratio at different engine operating conditions. U.S. Pat. No. 2,365,968 to Mason discloses a carburetor utilizing by-pass air controlled by valves that respond to ambient temperature and throttle valve setting. U.S. Pat. No. 2,733,903 to Harmon also discloses a carburetor for an internal combustion engine but adds supplementary air in response to higher engine speed conditions sensed as reduced pressure at the engine intake manifold.
U.S. Pat. No. 3,174,469 to Rappolt utilizes by-pass air in an internal combustion engine but to overcome engine operating problems associated with condensed fuel. U.S. Pat. No. 3,023,745 to Barnes introduces supplementary air into supplementary venture devices in response to sensed engine operating pressure conditions.
U.S. Pat. No. 3,669,424 to Shiobara discloses an engine carburetor which utilizes downstream air/fuel pressure to vary the effective area of a venturi during engine idle operation. U.S. Pat. No. 3,831,808 discloses a carburetor arrangement wherein by-pass air is provided downstream of a venturi in response to a sensed operating altitude condition.
U.S. Pat. No. 3,561,409 to August utilizes by-pass air in a carburetor for a liquid-fueled engine during idling and during coasting or down-hill (low load) engine operation to minimize the emission of unburned fuel in the engine exhaust. U.S. Pat. No. 4,686,951 to Snyder likewise varies the flow of air to an internal combustion engine during engine operation but controls by-pass air as a function of a detected oxygen level in the engine exhaust. U.S. Pat. No. 3,846,094 to Baverstock also utilizes by-pass air in a carburetor arrangement such by-pass air being modulated in response to a sensed demand manifold pressure condition.
U.S. Pat. No. 4,373,500 to Haynes discloses an engine supplemental air valve arrangement that is controlled directly in response to actuation of the engine accelerator (throttle valve control).
U.S. Pat. No. 4,479,466 to Greenway et al. teaches arrangements for alternatively introducing natural gas fuel into an internal combustion engine that normally utilizes liquid petroleum fuel. A somewhat similar arrangement but proposed for introducing propane as an alternate fuel for an internal combustion engine is disclosed by U.S. Pat. No. 4,494,515 to Brown. U.S. Pat. No. 4,497,304 to Wintrell et al. also discloses a dual fuel internal combustion engine system.
Lastly, U.S. Pat. No. 4,670,194 to Buford et al. also discloses a carburetor construction wherein two different air inlets are utilized. The first inlet introduces a minor amount of air at a carburetor location to intermix with fuel, and the second inlet introduces additional air at a location downstream of the first inlet.
Such prior art arrangements do not achieve the precision operating control for natural gas-fueled internal combustion engines that is achieved with the hereinafter described and claimed internal combustion engine control system.